The present invention relates to foam glass and more specifically to closed-cell foam silica.
Foam glasses are widely used in industry as lightweight, insulating materials. An example of a foam glass is Pittsburgh Coming FOAMGLAS (registered trademark). This material is composed of millions of individually sealed, micron-sized cells. This is called a closed-cell structure and offers a number of advantages over open cell foam glass structures.
Open cell foam glass structures can absorb moisture and other chemicals, greatly reducing the insulating efficiency of the foam glass. If an open cell foam glass absorbs a flammable material, it can act as a candle wick and can represent a serious fire hazard. If an open cell structure absorbs a corrosive chemical, the large surface area of the open cells can result in rapid corrosion.
Closed cell structures are largely impermeable to most liquids and gases and thus can be moisture-resistant and corrosion-resistant. For example, FOAMGLAS (registered trademark) can be used for outdoor and underground applications and can operate at temperatures up to 480 C.
Foam glasses are typically made using alkali metal silicate glasses. These glasses are fabricated by mixing, for example, alumina, boric acid, small silica particles and an alkali metal oxide with a cellulating agent to form a pulverulent homogeneous mixture as discussed in U.S. Pat. No. 4,192,664, the entire contents of which is herein incorporated by reference. The mixture is heated to a temperature that causes the cellulating agent to gasify, or vaporize, forming a foam glass with a cellular structure.
Although closed-cell foam glasses with high silica content are desirable, only open cell foam glasses having high silica content are typically reported as in U.S. Pat. No. 3,945,816, the entire contents of which is herein incorporated by reference. This foam silica was prepared by first fabricating a foam phase-separable alkali metal borosilicate glass, then heat treating the foam to separate the silica-rich and boron-rich phases, and finally leaching the boron-rich phase away using an acid etchant.
Open-cell foam silica can also be made using sol-gel techniques. This involves cooling of the liquid to cause gellation and phase separation followed by supercritical drying to remove the solvent and produce an open cell structure.